1. Field of the Invention
The present invention relates to a motor driving apparatus for driving a motor by supplying power from an AC power supply, and in particular to a motor driving apparatus having the function dynamically switching the converter operation mode of an AC/DC converter during powering operation of the motor.
2. Description of the Related Art
In a motor driving apparatus for a motor used to drive a machine tool, an industrial machine, a robot, or the like, a PWM converter method or a diode rectifier method is employed as an AC/DC conversion method to convert AC input power into DC power.
The PWM converter method is a method that brings the input current as close as possible to a sinusoidal waveform by applying PWM (pulse width modulation) control using power switching devices. While the PWM converter method has such advantages as being able to reduce harmonics of the input current and being able to make the output DC voltage variable, it has the disadvantage that the temperature increases due to the switching of the power switching devices is larger than in the diode rectifier method.
On the other hand, the diode rectifier method is a method that has been widely used and that performs rectification by passing the AC input from the power supply through a diode bridge circuit. While this method, contrary to the PWM converter method, has constraints such that the input contains a large amount of harmonics and that the output DC voltage is determined by the crest value of the AC input voltage, the temperature rise of the power switching devices is smaller than in the PWM converter method, because switching is not performed in the diode rectifier method.
In the regenerative operation in which the motor deceleration energy is returned to the power supply, the control that the PWM converter method performs is not much different from the control performed in the powering operation, and the switching devices need only be controlled so as to shift the phase of the current by 180 degrees relative to the phase of the voltage. On the other hand, in the case of the diode rectifier method, power regeneration is performed, not by the diodes, but by using devices such as IGBTs (insulated gate bipolar mode transistors) connected in reverse parallel with the diodes.
In the regeneration method commonly employed today, only two of the six switches are closed so that the regenerative current is caused to flow between the highest voltage phase and the lowest voltage phase in the three-phase power supply. This regeneration method is generally called the 120-degree conduction type power regeneration. The problem with this method is that since a large pulse-like current flows at once, the amount of noise is large and the output DC voltage abruptly changes, causing an undesirable side effect to the motor control.
Patent document 1 cited below discloses a technique that selects one or the other of the two operation modes, the diode rectifier mode or the PWM converter mode, when operating the motor in the powering mode. However, according to this technique, the operation mode once selected is fixed during the powering operation, so that the motor is operated only in the PWM converter mode or the diode rectifier mode, whichever is selected.
As described earlier, the PWM converter method has the problem that the temperature increases due to the operation of the power switching devices is large, while on the other hand, the diode rectifier method has the problems of a large amount of harmonics contained in the input current and the undesirable side effect due to the large current that flows when starting the regenerative operation. The PWM converter method may be employed in order to suppress the harmonic currents flowing to the power supply. However, if the load has increased during the PWM converter operation, the system has to be stopped in order to protect it from excessive increase in temperature.    Patent document 1: Japanese Unexamined Patent Publication No. H08-228490